The populations of four species of North American bumblebee have
declined, a new study has confirmed. The study also found that fungal
infections are more likely to plague these bees than other, more stable
bumblebee species.

Although perhaps not as dramatic as the sudden disappearance
of honeybees, a phenomenon dubbed colony collapse disorder, reports of
vanishing bumble bees have appeared in recent years in North America and
Europe.

Until now, however, the North American reports were isolated
and small-scale, according to Jeffrey Lozier, a study researcher and
postdoctoral researcher at the University of Illinois.

"What we wanted to do is say 'If you look at the entire
country, do these patterns hold up?'" Lozier said. "We picked these
target species because they sort of were canaries in the coal mine."

What they found added credence to worries of coast-to-coast
declines in some — but not all — bumble bees and more evidence of trouble for
pollinators that fertilize both wild plants and crops. The cause
remains unclear and may be complex.

"We need to keep a general view that pollinators seem
to be declining, but each bumblebee
species may be responding to different pressures that are causing
declines," said James Strange, a study author and a research entomologist
with the U.S. Department of Agriculture. "Not all the bees are
disappearing. It turns out there may be winners and there may be losers."

Bee collecting

Like honeybees, bumblebees
are employed to pollinate agricultural crops. Though they are less
numerous, their high-frequency buzz gives them an advantage, as the sound waves
free more pollen than a honeybee's buzz, and their large size allows them to continue
working in colder temperatures, according to Lozier.

The study focused on the western bumblebee, the American
bumblebee, the rusty-patched bumblebee, and the yellowbanded bumblebee.
Collectively, their ranges span the continental United States, while the
researchers trapped bumblebees at 382 locations across the country. They also
collected data on four species of bumblebee believed to be stable, and their
results indicated that these were indeed doing fine.

To get a sense of how the abundance and distribution of the
bees may have changed, the researchers looked at bumblebees preserved in museum
collections from 1900 to 1999, compiling a database of more than 73,000
historical specimens.

Their findings revealed that among the bees collected in the
field from 2007 through 2009, the four target species made up much smaller
portions of the total catches than they had historically. These changes in
relative abundance began to appear within the last 20 to 30 years, according to
the research published Jan. 3 (Monday) in the journal the Proceedings of the
National Academy of Sciences.

Based on this data, the researchers estimated that the four
target species had also seen their ranges decline. For example, the researchers
collected western bumblebees in the Rocky Mountains and the intermountain west
(between the Rockies and the Pacific coast), but it was largely absent from the
western portion of its historical range, closer to the Pacific coast.

The survey found only 22 rusty-patched bumblebees and 31
yellow-banded bumblebees.

Cause still unknown

The researchers also looked for infections by a fungus – Nosema bombi – and at the level of
genetic diversity among the eight bumblebee species. They found that 37 percent
of the western bumblebees they collected carried the fungus, and 15 percent carried
it among the American bumblebee, significantly higher infection rates than
those seen among the four stable species. Although there was evidence of higher
infection rates among the other two target species, too few were collected to
provide any definitive results.

While these findings indicate an association between the fungus
and declining populations, they don't necessarily show that the fungus is driving
the declines, Lozier said.

For the American and western bumblebee, researchers found
that populations also had less genetic diversity than stable species. (Once again, too few samples
were collected from other two species.) This is significant because genetic
diversity enables a population to respond to changing environments or novel
threats like disease, according to Lozier.

"The amazing thing we did find is gene flow appears
very high," Lozier said. For example, American bumblebees caught in Texas were
genetically indistinguishable from those from South Dakota, suggesting the bees
are reproducing (and spreading genes) across wide swaths of the United States.

"If gene flow is really this high, it could prove a
potential mechanism for the spread of the pathogen," he said.

The puzzle of
pollinator declines

The origin of the fungus and how it spread isn't completely
clear, but there are theories. It has been theorized that, after decimating
commercial bumblebee facilities in California, N. bombi escaped and became responsible for declines among wild
populations in the Pacific Northwest, according to these researchers.

The elevated N. bombi
infections among struggling bumblebees, and the possibility that the fungus was
introduced from Europe, calls to mind reports of other introduced fungal
pathogens decimating species — like the chytrid
fungus killing amphibians on multiple continents and Geomyces destructans, which is wiping
out some North American bats, they write.

Two papers published in December in the journal PLoS ONE explored
the plethora of infections faced by honeybees. One paper found that a certain
type of virus implicated in colony collapse disorder may be transmitted by
pollen and can infect other pollinators such as bumblebees and wasps. Another
study linked a viral-fungal tag team to the disorder. The researchers of those
studies found that infection becomes more lethal when the virus and fungus
infected the same bee together.

You can follow LiveScience
writer Wynne Parry on Twitter @Wynne_Parry.

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Author Bio

Wynne Parry

Wynne was a reporter at The Stamford Advocate. She has interned at Discover magazine and has freelanced for The New York Times and Scientific American's web site. She has a masters in journalism from Columbia University and a bachelor's degree in biology from the University of Utah.